Machinable coatings fabricated by slurry methods for use on ceramic matrix composites

ABSTRACT

A blade outer air seal has at least one internal surface and a layer atop the at least one internal surface. The layer has: a matrix comprising at least one of hafnium silicate (HfSiO4) and zirconium silicate (ZrSiO4), ytterbium disilicate (Yb2Si2O7); and barium magnesium alumino silicate (BMAS) or other alkaline earth aluminosilicate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/542,835, filed Aug. 16, 2019, which claims the benefit of provisionalapplication Ser. No. 62/764,907, filed Aug. 16, 2018.

BACKGROUND

The disclosure relates to ceramic matrix composite (CMC) components.More particularly, the disclosure relates to coatings for CMCcomponents.

Gas turbine engines typically include a compressor section, a combustorsection, and a turbine section. During operation, air is pressurized inthe compressor section and is mixed with fuel and burned in thecombustor section to generate hot combustion gases. The hot combustiongases are communicated through the turbine section, which extractsenergy from the hot combustion gases to power the compressor section andother loads.

The compressor and turbine sections of a gas turbine engine includealternating rows of rotating blades and stationary vanes. The turbineblades rotate and extract energy from the hot combustion gases that arecommunicated through the gas turbine engine. The turbine vanes directthe hot combustion gases at a preferred angle of entry into a downstreamrow of blades. An engine case of an engine static structure may includeone or more blade outer air seals (BOAS), which are typically formed ofmetal, that establish an outer radial flow path boundary for channelingthe hot combustion gases.

United States Patent Application Publication 20170350268 A1 (the '268publication), of McCaffrey, published Dec. 7, 2017, discloses a ceramicmatrix composite blade outer air seal segment. The disclosure of the'268 publication is incorporated by reference in its entirety herein asif set forth at length.

United States Patent Application Publication 20160215645 A1 (the '645publication), of McCaffrey, published Jul. 28, 2016, discloses a ceramicmatrix composite blade outer air seal segment. The disclosure of the'645 publication is incorporated by reference in its entirety herein asif set forth at length.

United States Patent Application Publication 20110219775 A1 (the '775publication), of Jarmon, et al., published Sep. 15, 2011, disclosesceramic matrix composite materials with a sprayed hard coating. Thedisclosure of the '775 publication is incorporated by reference in itsentirety herein as if set forth at length.

United States Patent Application Publication 20160332922 A1 (the '922publication), of Tang et al., published Nov. 17, 2016, discloses acoating for CMC comprising SiOC, BMAS, and SiO₂. Application is as aslurry. The disclosure of the '922 publication is incorporated byreference in its entirety herein as if set forth at length.

SUMMARY

One aspect of the disclosure involves blade outer air seal comprising atleast one internal surface formed within said blade outer air seal; anda layer atop the at least one internal surface comprising a matrixcomprising at least one of hafnium silicate (HfSiO₄), zirconium silicate(ZrSiO₄) and ytterbium disilicate (Yb₂Si₂O₇); and barium magnesiumalumino silicate (BMAS) and other alkaline earth aluminosilicate.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the at least one internalsurface comprises a ceramic matrix composite.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the layer is an outermostlayer.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the ceramic matrix compositeis a Si/SiC ceramic matrix composite.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the at least one internalsurface comprises at least one of a forward internal surface, an aftinternal surface and a ramped internal surface.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the layer is an outermostlayer.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the layer has a thickness of0.05 mm to 0.50 mm.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the layer has a BMAS or otheralkaline earth aluminosilicate concentration of 0.8% to 10.0% by weight.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include a method for using the bladeouter air seal, the method comprising exposing the blade outer air sealto a calcium magnesium aluminosilicate (CMAS)-forming environment.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include a method for coating a bladeouter air seal, the method comprising applying to at least one innersurface of said blade outer air seal a combination of at least one ofhafnium oxide (HfO₂), zirconium oxide (ZrO₂) and ytterbium oxide(Yb₂O₃); silicon carbide (SiC); silicon (Si) and barium magnesiumaluminosilicate (BMAS) or other alkaline earth aluminosilicate; andheating.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the applying is of one or moreslurries.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include a weight content of the BMASor other alkaline earth aluminosilicate relative to total combined HfO₂,Yb₂O₃, SiC, Si and BMAS or other alkaline earth aluminosilicate in theone or more slurries is 0.8% to 10.0%.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include the applying is via ultrasonicspray.

A further embodiment of any of the foregoing embodiments mayadditionally and/or alternatively include heating is in an oxidizingenvironment; and the heating is to a peak temperature of at least 1450°C.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings can be briefly described as follows:

FIG. 1 is a schematic, cross-sectional view of a gas turbine engine.

FIG. 2 is a partial view of the engine of FIG. 1 .

FIG. 3 is a partial longitudinal cutaway view of the engine.

FIG. 4 is a partial cross-sectional view of a blade outer air seal(BOAS) segment.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Examples are given below of the use of a material comprising a smallamount of barium magnesium aluminosilicate (BMAS) in hafnium silicate(HfSiO₄) and/or ytterbium disilicate (Yb₂Si₂O₇) (hereafter the“BMAS-containing material”). Particular illustrations involve specificuse and locations on a blade outer air seal (BOAS) segment whoseconfiguration is drawn from the '268 publication. The material may servevarious purposes such as smoothing a rough CMC, acting as a wearcoating, acting as a bond coating, acting as an adhesive or cement,acting as a filler, and the like.

FIG. 1 schematically illustrates a gas turbine engine 20 that includes afan section 22, a compressor section 24, a combustor section 26, and aturbine section 28. The exemplary illustrated engine is a two spoolengine as discussed below. Alternative engines might include anaugmenter section (not shown) among other systems or features. The fansection 22 drives air along a bypass flow path B while the fan sectionand compressor section 24 drive a core airflow in along a core flow pathC where air is compressed and communicated to the combustor section 26.In the combustor section 26, air is mixed with fuel and ignited togenerate a high pressure exhaust gas stream that expands through theturbine section 28 where energy is extracted and utilized to drive thefan section 22 and the compressor section 24.

Although the disclosed non-limiting embodiment depicts a turbofan gasturbine engine, it should be understood that the concepts describedherein are not limited to use with turbofans as the teachings may beapplied to other types of turbine engines; for example a turbine engineincluding a three-spool architecture in which three spoolsconcentrically rotate about a common axis and where a low spool enablesa low pressure turbine to drive a fan via a gearbox, an intermediatespool that enables an intermediate pressure turbine to drive a firstcompressor of the compressor section, and a high spool that enables ahigh pressure turbine to drive a high pressure compressor of thecompressor section. The concepts disclosed herein can further be appliedoutside of gas turbine engines.

The example engine 20 generally includes a low speed spool 30 and a highspeed spool 32 mounted for rotation about an engine central longitudinalaxis A relative to an engine static structure 36 via several bearingsystems 38. It should be understood that various bearing systems 38 atvarious locations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 thatconnects a fan 42 and a low pressure (or first) compressor section 44 toa low pressure (or first) turbine section 46. The inner shaft 40 drivesthe fan 42 through a speed change device, such as a geared architecture48, to drive the fan 42 at a lower speed than the low speed spool 30.The high speed spool 32 includes an outer shaft 50 that interconnects ahigh pressure (or second) compressor section 52 and a high pressure (orsecond) turbine section 54. The inner shaft 40 and the outer shaft 50are concentric and rotate via the bearing systems 38 about the enginecentral longitudinal axis A.

A combustor 56 is arranged between the high pressure compressor 52 andthe high pressure turbine 54. In one example, the high pressure turbine54 includes at least two stages to provide a double stage high pressureturbine 54. In another example, the high pressure turbine 54 includesonly a single stage. As used herein, a “high pressure” compressor orturbine experiences a higher pressure than a corresponding “lowpressure” compressor or turbine.

The example low pressure turbine 46 has a pressure ratio that is greaterthan about five (5). The pressure ratio of the example low pressureturbine 46 is measured prior to an inlet of the low pressure turbine 46as related to the pressure measured at the outlet of the low pressureturbine 46 prior to an exhaust nozzle.

A mid-turbine frame 58 of the engine static structure 36 is arrangedgenerally between the high pressure turbine 54 and the low pressureturbine 46. The mid-turbine frame 58 further supports bearing systems 38in the turbine section 28 as well as setting airflow entering the lowpressure turbine 46.

The core airflow is compressed by the low pressure compressor 44 then bythe high pressure compressor 52 mixed with fuel and ignited in thecombustor 56 to produce high speed exhaust gases that are then expandedthrough the high pressure turbine 54 and low pressure turbine 46. Themid-turbine frame 58 includes vanes 60, which are in the core airflowpath C and function as an inlet guide vane for the low pressure turbine46. Utilizing the vane 60 of the mid-turbine frame 58 as the inlet guidevane for low pressure turbine 46 decreases the length of the lowpressure turbine 46 without increasing the axial length of themid-turbine frame 58. Reducing or eliminating the number of vanes in thelow pressure turbine 46 shortens the axial length of the turbine section28. Thus, the compactness of the gas turbine engine 20 is increased anda higher power density may be achieved.

The disclosed gas turbine engine 20 in one example is a high-bypassgeared aircraft engine. In a further example, the gas turbine engine 20includes a bypass ratio greater than about six (6), with an exampleembodiment being greater than about ten (10). The example gearedarchitecture 48 is an epicyclical gear train, such as a planetary gearsystem, star gear system or other known gear system, with a gearreduction ratio of greater than about 2.3.

FIG. 2 is a perspective view of a portion 62 of the gas turbine engine20 of FIG. 1 . In this embodiment, the portion 62 is a portion of thehigh pressure turbine 54. It should be understood, however, that otherportions of the gas turbine engine 20 may benefit from the teachings ofthis disclosure, including but not limited to the fan section 22, thecompressor section 24, and the low pressure turbine 46.

In this embodiment, a rotor disk 64 (only one shown, although multipledisks could be axially disposed within the portion 62) is configured torotate about the engine central longitudinal axis A. The portion 62includes an array of rotating blades 66 (only one shown), which aremounted to the rotor disk 64, and arrays of static vane assemblies (notshown) on axial sides of the blades 66.

Each blade 66 includes a blade tip 66T at a radially outermost portionthereof. The rotor disk 64 is arranged such that the blade tips 66T arelocated adjacent a blade outer air seal (BOAS) assembly 68. The BOASassembly 68 may find beneficial use in many industries includingaerospace, industrial, electricity generation, naval propulsion, pumpsfor gas in oil transmission, aircraft propulsion, vehicle engines andstationary power plants.

The BOAS assembly 68 is disposed in an annulus radially between anengine case (such as an outer casing 70 of the engine 20) and the bladetips 66T. The BOAS assembly 68 includes a support structure 72 and aplurality of BOAS segments 74. The BOAS segments 74 may be arranged toform a segmented full ring hoop assembly that circumferentiallysurrounds the associated blades 66, which provides a sealing surface forthe blade tips 66T to prevent leakage of the core airflow over to theblades 66. For ease of reference, the individual BOAS segments 74 may bereferred to individually as a “BOAS segment” or simply a “BOAS.”

In this example, the support structure 72 includes a retention block 76fastened to the engine outer casing 70 by a fastener 78. The retentionblock 76 includes tapered arms 80, 82 on circumferentially opposed sidesthereof. The tapered arms 80, 82 in this example are rounded, and are tobe received within a corresponding curved end of a respective BOASsegment 74 (as will be explained below). FIG. 3 also shows a wedge seal79 sealing the inter segment gap and retained in a compartment in theinner diameter (ID) face of the retention block 76.

FIG. 4 illustrates an exemplary BOAS segment 74. In this example, theBOAS segment 74 is primarily made of a ceramic matrix composite (CMC)material. The BOAS segment 74 may include some non-CMC materials, suchas for the fillers or spacers, discussed below. CMC materials include aceramic matrix and a plurality of fibers suspended in that ceramicmatrix. The fibers can be a ceramic fibers, silicon carbide fibers,carbon fibers, or metallic fibers, as examples. As is discussed furtherbelow, additional fillers may be added in order to reorient plies/layersof the CMC to better resist fracture.

The BOAS segment 74 has a main body portion 90 having internal surfaces92. The internal surfaces 92 can include a forward internal surface 94,an aft internal surface 96 and a ramped internal surface 98 between theopposed forward internal surface 94 and aft internal surface 96.

The internal surfaces 92 can be covered by or replaced by a layer 100 ofthe BMAS-containing material. FIG. 4 shows such a layer 100 atop theinternal surfaces 92. The layer 100 is an outermost layer. The layer 100interfaces with the retention block and helps to provide wear resistanceand distribute loads. The exemplary layer 100 is formed in situ as acoating. An exemplary thickness is 0.05 mm to 0.50 mm, morespecifically, 0.07 mm to 0.15 mm at least locally/peak, optionallyfeathering or tapering out. Exemplary coating properties may have thecoating harder than the CMC to reduce pressure concentrations in theCMC. The coating may be sprayed to a greater thickness and then grounddown. An exemplary thickness after being machined down can comprise 0.01mm to 0.015 mm.

FIG. 4 shows a second layer 102 of the BMAS-containing material. Thelayer 102 is used as a bondcoat to transition from the CMC to a layer92.

Exemplary application techniques for the BMAS-containing materialinvolve applying a precursor slurry such as dipping, painting, airspraying, and ultrasonic spraying.

The slurry is a precursor slurry to the extent that it comprisescomponents of the hafnium silicate and/or zirconium silicate and/orytterbium disilicate which are reacted post spraying. The examples belowinclude the BMAS in the slurry. Exemplary slurries are aqueous slurrieswhere the carrier is distilled or deionized water. The exemplarycomponents for subsequent reacting include silicon and carbon in theform of silicon carbide or silicon along with an oxide of the hafnium orzirconium or ytterbium (respectively hafnium oxide (HfO₂) or zirconiumoxide (ZrO₂) or ytterbium oxide (Yb₂O₃). A post spray heating reacts thesilicon carbide or silicon with the oxide to form the hafnium silicateand/or zirconium silicate and/or ytterbium disilicate. The heating isperformed in an oxidizing atmosphere (e.g., containing pure oxygen, air,or a mixture) to provide the additional required oxygen for the reaction(e.g., HfO₂+SiC+2O₂→HfSiO₄+CO₂ or HfO₂+Si+2O₂→HfSiO₄).

Exemplary post spray heating is to a temperature of about 2700° F.(1482° C.) more broadly, a range may be at least 1350° C.) at least2500° F. (1371° C.) or at least 1400° C. or at least 1450° C. or anexemplary 2500° F. (1371° C.) to 2700° F. (1482° C.) or 1350° C. to1500° C. Exemplary time may be at least 12 hours or an exemplary 4-48hours, more particularly 12-30 hours or about 24 hours.

The slurries may be prepared by mixing powders of the constituents inthe carrier liquid. Tables I&II below provide exemplary ranges ofconstituents.

TABLE I Powder Range 1 Range 2 Example 1 Example 2 Example 3 Constituentsize Wt % Vol %* Wt % Vol* % Wt % Vol %* Wt % Vol %* Wt % Vol %* HfO₂1-2 um 35-60 40-60 30-70 40-80 55 61 49 61 40 45 SiC 1-2 um  5-20 30-50 5-20 15-55 10 35  9 33 15 50 BMAS −325 mesh  1-10  5-20 0.1-10   1-20 1  5  2 10  1  5 (<45 um) H₂O N/A ~40 75-85 ~40 75-85 46 83 42 80 40 83*Volume percent of particulates does not include the carrier (water) inthe denominator.

TABLE II Powder Range 1 Range 2 Example 1 Example 2 Example 3Constituent size Wt % Vol %* Wt % Vol* % Wt % Vol %* Wt % Vol %* Wt %Vol %* Yb₂O₃ 1-2 um 35-60 40-60 30-70 40-80 40 45 48 63 56 78 SiC 1-2 um 5-20 30-50  5-20 15-55 15 50  9 33  4 18 BMAS −325 mesh  1-10  5-200.1-10   1-20  1  5  1  5  1  5 (<45 um) H₂O N/A ~40 75-85 ~40 75-85 4683 42 80 40 83 *Volume percent of particulates does not include thecarrier (water) in the denominator.

Table III below shows test data discussed below. Cracking anddelamination are observed at at high ratio of Yb to Si. Some crackingbut no significant delamination occurred at lesser ratio and it isbelieved this may be ameliorated via variation in coating thickness.Coatings were about 300-400 micrometers.

TABLE III Test 1 Test 2 Test 3 Test 4 Test 6 Reactive Hf Hf Yb Yb Ybelement (RE) Molar 0.5 1.0 1.0 2.6 0.5 RE/Si Wt. % 7   5   5   5   5  BMAS Result No Cracking No Cracking Cracking cracking and no crackingand and no or delami- delami- or delami- delami- delami- nation nationnation nation nation

Exemplary molar ranges of Hf, Yb, or their combination to Si are0.5-2.0, more particularly 0.5-1.5.

Exemplary application is via AccuMist™ Spray Shaping System of Sono-TekCorporation, Milton N.Y.

In an exemplary test for resistance to calcium magnesium aluminosilicate(CMAS) attack, a sample of the coating material on a CMC substrate wasprepared by initially marking out a 1 cm by 1 cm square on the coatingand applying 0.010 g of CMAS powder via sprinkling onto that markedarea. A spatula was used to spread the powder over the area and then 2-5drops of isopropanol was applied to the powder (until powder appearedfully wetted). The spatula was used to mix and evenly distribute theCMAS-isopropanol slurry.

The sample was allowed to air dry for a few minutes and then placed in acrucible and covered. The crucible is placed in a furnace and heated ata desired temperature (2700 F in the tests) for a desired time (100hours).

The post heating samples were observes to detect the presence ofspalling (Table III above).

The results for the examples above were that a beneficial range of BMASis 1.0% to 10.0%, more particularly, 1.0% to 8.0% or 2.0% to 7.0%. Loweramounts are believed to hinder densification, exposing the substrate togreater oxidation and exposing the coating to greater CMAS infiltration;greater amounts are believed to increase chances of adverse reactionbetween the BMAS and the Silicon carbide of the substrate.

The use of “first”, “second”, and the like in the following claims isfor differentiation within the claim only and does not necessarilyindicate relative or absolute importance or temporal order. Similarly,the identification in a claim of one element as “first” (or the like)does not preclude such “first” element from identifying an element thatis referred to as “second” (or the like) in another claim or in thedescription.

Where a measure is given in English units followed by a parentheticalcontaining SI or other units, the parenthetical's units are a conversionand should not imply a degree of precision not found in the Englishunits.

One or more embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made. For example, whenapplied to an existing baseline configuration, details of such baselinemay influence details of particular implementations. Accordingly, otherembodiments are within the scope of the following claims.

What is claimed is:
 1. A method for coating a blade outer air seal, themethod comprising: applying to at least one inner surface of said bladeouter air seal a combination of: at least one of hafnium oxide (HfO₂),zirconium oxide (ZrO₂) and ytterbium oxide (Yb₂O₃); silicon carbide(SiC) or silicon (Si) and barium magnesium aluminosilicate (BMAS) orother alkaline earth aluminosilicate, wherein said barium magnesiumalumino silicate (BMAS) or other alkaline earth aluminosilicate includesa concentration of 0.8% to 10.0% by weight; and heating the combinationapplied to the blade outer air seal.
 2. The method of claim 1 wherein:the applying is of one or more slurries.
 3. The method of claim 2wherein: the weight concentration of the BMAS or other alkaline earthaluminosilicate relative to the concentration in the one or moreslurries is 0.8% to 10.0%.
 4. The method of claim 2 wherein: theapplying is via ultrasonic spray.
 5. The method of claim 1 wherein: theheating is in an oxidizing environment; and the heating is to a peaktemperature of at least 1450° C.